An optical communication system is a system for transmitting and receiving optical signals by way of optical fiber, and an optical receiver is one component of an optical communication system. In an optical receiver, typically signal light that enters into the optical receiver by way of optic fiber is condensed and collected in a photoelectric converter by way of a lens or the like, and converted to an electric signal by the photoelectric converter. The electric signal that is obtained by converting the incoming light undergoes processing such as amplification and/or the like, and then outputted from the optical receiver.
The wavelength of signal light that is used in an optical communication system is set according to standards. For example, in ITU-T (International Telecommunication Union Telecommunication Standardization Sector) G.693, the use of light having wavelengths of 1.31 um (micrometers) and 1.55 um is regulated for short-range high-speed signal transmission.
When light having different wavelengths is used for optical communication, the optical receiver preferably receives light of any wavelength with good sensitivity. However, the index of refraction for lenses made using typical glass material differs for each wavelength, so that when light having different wavelengths passes through a lens, chromatic aberration occurs. When the coupling efficiency between optical fiber and a photoelectric converter for light having a certain wavelength is improved in a state in which chromatic aberration has occurred, the coupling efficiency of light of other wavelengths decreases. In a state such as this wherein chromatic aberration has occurred, the coupling efficiency depends on the wavelength of light, so that the sensitivity of the optical receiver also depends on the wavelength of light.
Various technologies have been proposed for correcting chromatic aberration in order to make the coupling efficiency the same for light of all wavelengths and to improve the sensitivity for light of all wavelengths. For example, Unexamined Japanese Patent Application Kokai Publication No. H5-34642 discloses technology for correcting chromatic aberration by combining a convex lens and a concave lens.
However, in the technology disclosed in Unexamined Japanese Patent Application Kokai Publication No. H5-34642, a plurality of lenses is combined, resulting in a problem in that the construction of the optical receiver becomes complex. Moreover, in constructing an optical receiver by combining a plurality of lenses, there is a problem in that much work is required in order to perform optical-axis alignment of the lenses.
Taking the situation above into consideration, it is the objective of the present invention to provide an optical receiver having simple construction that is capable of correcting chromatic aberration.